Offshore petroleum operations frequently employ a shack or shed on the drilling/production platform which houses any number of machines, equipment and other electrical devices necessary for the maintenance, operation and safety of the drilling platform. Any number of inflammable hydrocarbon vapors are present on the drilling/production rig and can easily be ignited or combusted by electrical arcing, cigarette lighting or welding operations. In an attempt to eliminate the above-mentioned fire hazards, it has become common practice to maintain the interior of the shed housing the equipment and machinery at a pressure greater than the pressure outside the shack. Therefore, the greater pressure inside the shack insures that none of the inflammable vapors can enter into the shack and thereby become combusted or inflamed.
In an attempt to increase further the safety of the drilling platform environment, an automatic shut-off switch to all the associated machinery has been connected to a differential pressure gauge such that when the pressure inside the shack relative to that outside the shack is reduced below a certain level, for example equal to that of the outside pressure, all the associated equipment is immediately cut off. In order to compare accurately the pressure inside the shack with that outside the shack, static pressure must be measured in both instances. In the past, static air inlet ports have been projected from within the shack through the roof or wall thereof in order to communicate with the outside environment. Consequently, in a no wind situation accurate readings are obtained. The presence of a wind, however, causes the sum of both the dynamic and static pressures to be read erroneously as static pressure. If a wind develops such that the static air port is on the upwind side of the shack, the actual pressure measured is greater than the true static pressure inasmuch as both static and dynamic pressures are being read on the pressure gauge. A false alarm arises, because the outside pressure is erroneously read high, thus reducing the measured ratio of the inside pressure to the outside pressure to a smaller fraction and giving a false "low inside pressure" reading. Periods of up to three hours are frequently required to reactivate all the equipment which has needlessly been cut off.
Similarly, if the static air port is located downwind, a low pressure area surrounding the static air port frequently occurs and pressures lower than the true static pressure will be read outside the shack. In such a situation the denominator (outside pressure) is reduced thereby increasing the ratio of inside to outside pressure. Accordingly, the actual inside pressure can fall below the safe level and produce a hazardous situation.